<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
</head>

<body>
  <script>
    function foo() {
      var x = 20;
      var y = 10;

      function child() {
        var m = 5;

        return function add() {
          var z = 'this is add';
          return x + y;
        }
      }

      return child();
    }
    foo()();
  </script>
</body>

</html>